High volume electrophotographic copiers, duplicators and printers are presently manufactured to produce from 50,000 to one million images a month and more at high speed. Some can do duplex at full machine speed and "highlight" color at a reduced speed.
For example, U.S. Pat. No. 4,078,787, Burlew et al, issued Mar. 14, 1978, shows a commercially known electrophotographic copier in which a photoconductive belt is entrained around a series of rollers. Toner images are formed on the belt electrophotographically and are transferred to a receiving sheet which is brought into contact with and carried by the belt for a portion of its path. Copiers and printers similar to this apparatus presently do duplex imaging at over 100 images a minute.
In such apparatus, the belt is entrained about a one-inch roller just after the transfer station. The receiving sheet has a tendency not to follow the photoconductive belt as it goes around the small roller and is picked up by a transport belt, one edge of which is positioned just above the small roller. The transport belt holds the non-image bearing side of the receiving sheet and transports it away from the photoconductive belt to a fixing device, for example, a roller fuser.
U.S. Pat. No. 4,714,939, Ahern et al, issued Dec. 22, 1987, shows a high volume copier not yet adopted commercially. In order to do duplex copying with a straight receiving sheet path, an intermediate transfer roller or belt is positioned in transfer relation with the photoconductive belt. A first toner image is transferred to the intermediate member and the receiving sheet is fed between the photoconductive belt and the intermediate member. The first image is transferred to the topside of the receiving sheet and a second toner image is transferred to the bottom side of the receiving sheet. With this approach, duplex images can be formed on a receiving sheet with the receiving sheet passing through a straight paper path. Because toner images are electrostatically transferred in opposite directions to opposite sides of the same sheet, the intermediate roller or belt is positioned to separate from the image member before the toner image is transferred to the receiving sheet from the photoconductive belt. This reduces the tendency of the other toner image to be transferred back to the intermediate under influence of the electrostatic field transferring the toner image from the photoconductive belt. See also U.S. Pat. No. 4,688,925, Randall, issued Aug. 25, 1987.
A number of references describe a process for making two (or more) color images by creating an electrostatic image and toning the electrostatic image in the presence of a previously created toner image of a different color. U.S. patent application Ser. No. 07/341,452 to Ahern, filed Apr. 21, 1989, and entitled "Color Duplex Reproduction Method and Apparatus", discloses using that process with an intermediate belt or roller to do multicolor duplex toner images using a straight paper path.
The duplex copiers and printers described above are designed to operate at high speeds and high volumes. Such apparatus generally has a large capacity paper supply, a heavy-duty fuser and are quite reliable despite very high volume applications.
U.S. Pat. No. 4,531,828, Hoshino, issued Jul. 30, 1985, and U.S. Pat. No. 4,580,889, Hiranuma et al, issued Apr. 8, 1986, are representative of a number of references which show the use of four separate photoconductive drums for creating single color toner images which are then transferred in registration to a receiving sheet or an intermediate member to a form multicolor image.
Four-color printers and copiers are generally relatively slow speed. However, they still require a relatively heavy-duty fuser in order to fuse images made up of four different toners.